Update gRPC to v1.30.2 and google-protobuf to v3.12.4

This improves the Gitaly gRPC error message. These upgrades were done in !2319 (merged), but they were reverted in !2339 (merged).

Since then, we have fixed CentOS 6 builds to use a newer gcc (omnibus-gitlab!4466 (merged)).

For musl-based installations, google-protobuf will install but not run. As mentioned in https://github.com/protocolbuffers/protobuf/issues/2335#issuecomment-579913357, this workaround will allow google-protobuf to build:

bundle config build.google-protobuf --with-cflags=-D__va_copy=va_copy
BUNDLE_FORCE_RUBY_PLATFORM=1 bundle install

A possible fix for this is in https://github.com/protocolbuffers/protobuf/pull/7773.

Closes: #2786 (closed)